1. Field of the Invention
This invention relates to the field of ground detectors. Specifically it relates to the class of ground detector known as an active dynamic ground detector.
2. Description of the Prior Art
The need for ground fault detection in electric power distribution is as old as the power distribution art. Many devices have been proposed to aid the power supplier in sensing and finding such faults. In recent years, hospitals have found it desirable to use isolated power supplies for use in operation rooms, etc. so as to avoid as much as possible accidents resulting from electrical sparks, shocks, etc. To indicate that the isolated power supplies are actually free of grounds, line isolation monitors have been developed. A regulatory agency, National Fire Prevention Association (NFPA - 56, 1956), has set up standards for ground isolation in hospitals. To comply with the standards, a total leakage no greater than .002 amperes from line to ground is allowable. Included in this leakage figure is the capacitative leakage of the power supply itself, and the power required to drive the ground indicating device. With the limits on leakage as set up by NFPA, it is desirable that the driving power of the ground sensor be kept at as low a value as possible so that the growth of leakage below the allowable maximum can be watched. In general, the leakage contribution of many presently known ground fault detectors is of the order of 0.0005 amperes. With the limit of 0.002 amps total leakage for a system, the relatively high current draw of such devices prevents any accurate reading of the possible fault currents.
In a typical hospital installation, power is fed from the utilities lines to an isolation transformer. Non-grounded power lines from the isolation transformer feed one or more operating rooms, intensive care wards, etc. A line isolation monitor is connected to the non-grounded lines at any suitable spot. In a preferred installation, a signal is provided at each use area to advise that the power line is either: (a) free from grounds or (b) that a ground fault exists.
Presently known line isolation monitors can be classified into groups. One such classification is:
1. Passive static ground detectors PA1 2. Active static ground detectors PA1 3. Passive dynamic ground detectors PA1 4. Active dynamic ground detectors
In this classification, passive indicates that the device is operated by line power and active indicates that the device supplies a fault reading supply differing from the line power. Static refers to a system continuously connected to both sides of the power supply by a suitable bridge. Dynamic refers to a system which alternately senses one and then the other side of the isolated power line.
Line isolation monitors or ground detectors of groups 1, 2 and 3, are well known in the art. The general theories of fault detection have been completely worked out over the course of years. It is important to note that a single fault from one side of the power line to ground is comparatively easy to identify. However, when faults occur simultaneously on both sides of the power line and particularly when such double faults are identical or balanced in nature (such as the mid-point grounding of RFI suppressing capacitors) it becomes much more difficult to obtain a fault indication. In static systems, it is practically impossible to sense all types balanced faults.
The reliability of any safety device is of greatest importance and must be continually considered by the designer of safety apparatus. The designer must not only strive to use reliable parts and circuitry but he must also consider the results of component failure no matter how remote the chances are of such failure. Circuitry having a minimum of components operating in straight forward modes is preferred to more elaborate circuitry having a greater number of parts and having greater probability of failure.